(1) Field of the Invention
The present invention relates to a rotator driving device for rotationally driving a rotator, such as a photosensitive drum provided in a copier, color printer, facsimile, or the like, and also relates to an image forming apparatus using the rotator driving device and a method of rotating such a rotator.
(2) Related Art
In general, a high degree of uniformity is required to rotate a rotator. The reason why the high-degree uniformity is required is given for a case of a photosensitive drum provided as a rotator in, for example, a copier.
The photosensitive drum is rotated in one direction, and a laser beam scans the surface of the photosensitive drum in the direction of the axis of rotation i.e. the main scanning direction) at every scanning cycle. As a result of scanning, an electrostatic latent image is formed on the surface of the photosensitive drum. If the rotational speed of the drum is unstable, that is, if the drum has nonuniformity in rotation, nonuniformity accordingly occurs in the distances between the scanning lines. This causes inconsistency in the print density on a reproduced image and so deteriorates the image quality. For this reason, a high level of uniformity is required in the rotational speed of the photosensitive drum.
The nonuniformity in rotation is caused by various factors. It may be caused due to eccentricity of the photosensitive drum and thus occur in a cycle of one rotation of the drum. The nonuniform rotation at a low frequency may be caused by torsion of a motor shaft and a load shaft. Meanwhile, the nonuniform rotation at a high frequency may be caused by improper engagement of gearwheels or a timing belt.
To raise the level of uniformity in rotation, a reduction device having a planetary roller instead of gearwheels or a timing belt has been used. By means of this reduction device, the nonuniformity in rotation at a high frequency can be eliminated. This technique is disclosed in Japanese Laid-Open Patent Application Nos. 5-53381 and 5-180290, for example.
The nonuniformity in rotation at a high frequency that is caused by the improper engagement of the gearwheels used as a reduction device can be improved by a rotator driving device that has a planetary roller. However, there is a possibility that the nonuniformity in rotation at a low frequency would increase due to factors, such as a skid of the planetary roller. This problem occurs not only to a photosensitive drum provided in an image forming apparatus. It commonly occurs to other kinds of rotator driving devices that each have a reduction device with a planetary roller and that require uniformity in the rotational speed.
Additionally, a rotator driving device conventionally has to be provided for each rotator, such as a photosensitive drum or developing roller. Therefore, these rotator driving devices to be equipped in an apparatus, such as a copier, occupy a large space, thereby making hard to manufacture the apparatus as compact as possible.
To realize a compact rotator driving device, Japanese Laid-Open Patent Application No. 4-245261, for instance, discloses a technique of rotationally driving a plurality of rotators using only one driving source. To be more specific about this technique, a servomotor that has a reduction device with a planetary roller directly drives one of the provided rotators, and then a rotational force of the servomotor is sequentially transferred to the other rotators via idler rollers.
In this case, however, attention should be paid to that the rotators are mechanically connected to each other via the idler rollers, meaning that the neighboring rotator and idler roller are in contact with each other. With this construction, if low frequency elements, such as nonuniformity in rotation or vibration, occur to one of the rotators, these elements may be transferred to the other rotators via the idler rollers. In addition, if such low frequency elements occur to one of the idler rollers, the following rotators would be adversely affected by this increased nonuniformity. Thus, the aim of realizing a driving device that has a high degree of precision cannot be achieved.
The first object of the present invention is to provide a novel rotator driving device and an image forming apparatus using the rotator driving device. The rotator driving device of the present invention eliminates nonuniformity in rotation at a low frequency that occurs to a rotator rotated by a reduction device with a planetary roller, so that the rotational speed of the rotator can be more stable. A photosensitive drum provided as a rotator in the image forming apparatus is rotated at a constant speed using the rotator driving device, so that high-quality images can be reproduced.
The second object of the present invention is to provide a novel rotator driving device, an image forming apparatus using the rotator driving device, and a novel method of driving a rotator. The rotator driving device of the present invention can respond to the trend toward size reduction, and prevent nonuniformity in rotation or mechanical vibration of a rotator from adversely affecting another rotator so that each rotator can rotate at a constant rotational speed.
The first object of the present invention can be achieved by a rotator driving device that rotationally drives a rotator, the rotator driving device being made up of: a motor; a planetary-roller reduction unit that outputs a rotational speed that is reduced with respect to a rotational speed of the motor, and transfers a rotational drive force of the motor to the rotator; a speed detector that detects a rotational speed of the rotator; and a controller that controls the rotational speed of the motor in accordance with a detection result obtained by the speed detector.
With this construction, the rotational drive force of the motor serving as the rotational driving source is transferred to the rotator via the planetary-roller reduction unit. Also, the controller controls the rotational speed of the motor in accordance with the detection result obtained by the speed detector. Consequently, nonuniformity in rotation at high and low frequencies is eliminated, so that the rotational speed of the rotator can be maintained constant. The second object of the present invention can be achieved by a rotator driving device that is provided in an image forming apparatus and that drives first and second rotators provided in the image forming apparatus, the rotator driving device being made up of: a driving unit that includes a motor and supplies a rotational drive force of the motor to the first rotator; a speed detector that detects a rotational speed of the first rotator; a controller that controls a rotational speed of the motor in accordance with a detection result obtained by the speed detector; and a drive branching unit that branches the rotational drive force of the motor and transfers the branched rotational drive force to the second rotator. It should be noted here that xe2x80x9cbranchxe2x80x9d used in the present specification does not mean xe2x80x9cseparatexe2x80x9d or xe2x80x9cdivide.xe2x80x9d To be more specific, even after a rotational drive force branches off using the drive branching unit, the branched force is still the same as the original rotational drive force in strength.
By means of this construction, the rotational drive force of the motor is transferred to the first rotator via the speed reducer while the rotational drive force branched by the drive branching unit is transferred to the second rotator. As such, the rotational drive force can be transferred to a plurality of rotators using only one rotational driving source. This leads to space saving, and the image forming apparatus can be manufactured compact. The rotational speed of the motor is controlled in accordance with the detected rotational speed of the first rotator. Thus, if nonuniformity in rotation occurs to the second rotator, the nonuniformity is prevented from being transferred to the first rotator. As a result, the first rotator is always rotationally driven at a constant speed.
The second object of the present invention can be also achieved by a rotator driving device that drives first and second rotators, the rotator driving device being made up of: a motor that supplies a rotational drive force to the first rotator; and a drive branching unit that branches the rotational drive force, the branched rotational drive force being used for driving the second rotator, wherein the drive branching unit has a main rotating member and a slave rotating member, the main rotating member being set coaxial with the first rotator and so rotating as the first rotator rotates and the slave rotating member being connected to the second rotator and rotating together with the main rotating member owing to a magnetic action exerted between the main and slave rotating members.
With this construction, the rotational drive force can be transferred to a plurality of rotators using only one motor. This leads to space saving and an image forming apparatus including the rotator driving device can be manufactured compact. The drive branching unit is composed of main and slave rotating members, the slave rotating member rotating together with the main rotating member owing to the magnetic action exerted between the main and slave rotating members. By means of this construction of the drive branching unit, nonuniformity in rotation occurring to a rotator can be absorbed more as compared with a case where the rotational drive force branches off using a gear or the like. Consequently, the nonuniformity in rotation can be reliably prevented from adversely affecting another rotator.